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20th November 2011 @ 06:12
Vilsmeier Haack synthesis of 2,5-dimethyl-1H-phenyl pyrrole-3-carboxaldehyde.


DMF (1.5 mL, 19.4 mmol, 6.5 equiv.) was stirred under a nitrogen atmosphere in an ice-bath. Phosphoryl chloride (0.308 mL, 3.3 mmol, 1.1 equiv.) was added and the reaction stirred for 30 minutes. Reaction mixture was a very pale yellow. A solution of 2,5-dimethyl-1H-phenyl-pyrrole (514 mg, 3 mmols, 1 equiv.) in DMF (2 mL) was added dropwise over 1 minute. After 5 minutes, the flask was removed from ice and reaction stirred for a further 55 mins. TLC at 1 hr showed reaction to be complete. Reaction mixture was poured onto ice (50 mL) and 1M NaOH added (pH 11), until adjusted to pH 6. Total volume approximately 50 mL after ice has melted and was pH 3 the next day. 20% NaOH was added to achieve pH 11 and flask bathed in a brine ice-bath for 20 minutes. Product was filtered and washed with water to produce a wet brown paste. Product was dissolved in MeCN and concentrated under a nitrogen atmosphere. Brown-green solution with slight precipitation was observed overnight. Product was then extracted using a mixture of EtOAc (2 × 20 mL), Et2O (20 mL), water (20 mL) and brine (20 mL). Product was concentrated under reduced pressure and cooled in refrigerator overnight to crystallise. Product was dissolved in EtOH (3 mL) in a water bath at 40°C. Solution was cooled in a brine ice bath and filtered. Filter cake was washed with water (3 × 10 mL) and product dried under vacuum to give a grey free-flowing powder (407 mg, 64%).

Collected data:2,5-dimethyl-1H-phenyl pyrrole-3-carboxaldehyde

mpt: 88-90oC
1H-NMR (300 MHz, CDCl3): δ 9.88 (s, 1H), 7.55-7.50 (qn, 3H), 7.19, 7.22 (t, 2H), 6.39 (s, 1H), 2.28 (s, 3H), 1.99 (s, 3H)
13C-NMR (75 MHz, CDCl3): δ 185.29, 138.85, 136.99, 131.01, 129.58, 128.91, 127.97, 121.91, 105.84, 12.65, 11.22
m/z (ESI +/-): 200 [M+H]+, 100%
IR: 668.92, 703.93, 801.29, 1422.13, 1650.27

For data files, refer to Vilsmeier Haack Synthesis of 2,5-dimethyl-1-phenyl-1H-pyrrole-3-carbaldehyde (LMW 4-1)

doi: 10.1002/cmdc.200600026
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